This invention relates to communications systems and particularly but not exclusively to cellular communications systems.
In a cellular communications system such as a GSM (Global System for Mobile telecommunication) system, a mobile unit is arranged to communicate with a base station via a radio frequency. The base station is able to accurately achieve the correct frequency for transmission, by virtue of very accurate (and expensive) tuned crystals and circuits, including temperature compensating circuits.
A problem with this arrangement is that the mobile is typically equipped with a less accurate crystal and a simple tuning circuit, and the crystal must be tuned in order that the correct frequency can be received. A known solution to this problem is shown in FIG. 1, where a demodulator arrangement 5 of a mobile device has a frequency estimation scheme. An antenna 10 of the demodulator arrangement 5 receives a dedicated calibration signal, which is periodically transmitted in bursts by the base station.
A first Radio Frequency (RF) Voltage Controlled Oscillator (VCO) 20 and a second Intermediate Frequency (IF) VCO 30 demodulate the calibration signal in two stages, using oscillations from a quartz crystal 25. The VCOs 20 and 30 provide phase (IP) and quadrature (IQ) signals to a sigma-delta converter 40 which quantises the IP and IQ signals. The quantised signals are then passed through a Low-Pass Filter (LPF) 50 before being fed to software modules 60. The filtered, quantised IP and IQ signals represent a measured value. The dedicated calibration signal contains a known component which, when demodulated at the correct frequency by the VCO""s 20 and 30, will produce a sinusoidal signal having a predetermined value. The software modules 60 compare the predetermined value with the measured value in order to derive an error value. This error value is fed to a Digital-to-Analogue Converter (DAC) 70 which converts the error value to a voltage which is used by a varicap circuit 80 to to fine-tune (or calibrate) the crystal 25 to the desired correct frequency. In this way the demodulator arrangement 5 of the mobile device is calibrated to the correct frequency.
However, a further problem occurs in that the dedicated signal must itself be received and demodulated correctly in order to provide the frequency estimation scheme, and this requires that the mobile device has a crystal with at least a certain accuracy, and thereby a certain minimum cost. This is because if the accuracy of the crystal is too poor, the pre-corrected frequency of the demodulator arrangement 5 will be so different to the predetermined frequency that the dedicated calibration signal will not be successfully received and demodulated.
This invention seeks to provide a communications system, mobile device and method which mitigate the above mentioned disadvantages.
According to a first aspect of the invention there is provided a communications system having an air interface, and comprising: a base station arranged for transmitting a modulated signal having a predetermined frequency over the air interface; and a mobile device arranged for receiving the modulated signal, the mobile device comprising: a frequency demodulator having a demodulation frequency, the frequency demodulator being arranged for demodulating the modulated signal; and, frequency correction means arranged for tuning the demodulation frequency to the predetermined frequency by providing a main difference estimation between the predetermined frequency and the demodulation frequency using a dedicated narrowband portion of the modulated signal, wherein the frequency correction means is further arranged to perform a preliminary difference estimation between the predetermined frequency and the demodulation frequency using a broadband portion of the modulated signal, before the main difference estimation.
According to a second aspect of the invention there is provided a mobile communications device arranged for receiving a modulated signal having a predetermined frequency from a base station over an air interface, the mobile device comprising: a frequency demodulator having a demodulation frequency, the frequency demodulator being arranged for demodulating the modulated signal; and, frequency correction means arranged for tuning the demodulation frequency to the predetermined frequency by providing a main difference estimation between the predetermined frequency and the demodulation frequency using a dedicated narrowband portion of the modulated signal, wherein the frequency correction means is further arranged to perform a preliminary difference estimation between the predetermined frequency and the demodulation frequency using a broadband portion of the modulated signal, before the main difference estimation.
According to a third aspect of the invention there is provided a method of providing frequency correction for a frequency demodulator having a demodulation frequency, the frequency demodulator being arranged to receive a modulated signal having a predetermined frequency, the method comprising the steps of: providing a preliminary difference estimation between the predetermined frequency and the demodulation frequency using a broadband portion of the modulated signal; tuning the demodulation frequency to the predetermined frequency using the preliminary difference estimation; providing a main difference estimation between the predetermined frequency and the demodulation frequency using a dedicated narrowband portion of the modulated signal, and, further tuning the demodulation frequency to the predetermined frequency using the main difference estimation.
In this way a mobile device is able to utilise an inexpensive crystal oscillator, with a more relaxed accuracy requirement than would otherwise be necessary. This is because the dedicated narrowband portion of the modulated signal is not used during the preliminary difference estimation.